The present invention relates to a circuit which protects a data driver from damage due to an over current.
The data driver is used as an output stage of a logic apparatus such as, for example, a digital word generator which applies digital words to a PUT (product under test) for a measurement or troubleshooting. The output impedance of the data driver must be low for applying effectively the output digital words to the PUT. Sometimes, an undesirable fault happens due to an operator's mistake or a malfunction of the PUT, i.e., the output end of the data driver is shorted to a high level portion of the PUT when the output is a low level, or the output end thereof is shorted to a low level portion of the PUT when the output is a high level. In this instance, a large or over current flows through the data driver because of the low input impedance thereof, and the driver may be damaged. Therefore, a protection circuit is necessary to protect the driver from the damage due to the over current.
One conventional protection circuit is disclosed in U.S. Pat. No. 3,495,179. This circuit comprises an operational amplifier having a non-linear feedback network and an overload indicator, wherein the feedback network reduces the gain of the operational amplifier in response to input signals having a magnitude exceeding a threshold overload value. However, this circuit is not proper for the above data driver, because the input logic levels are predetermined and do not exceed the threshold overload value. Thus, the prior art circuit cannot protect the data driver from the damage due to the short of the data driver's output.
U.S. Pat. No. 3,849,734 discloses another protection circuit, wherein a comparator compares an output voltage from an amplifier with a predetermined voltage, and shunts input terminals of the amplifier when the output voltage exceeds the predetermined voltage. This circuit, however, cannot protect the data driver from the damage due to the short of the output terminal, because the logic high and low levels are predetermined, and the voltage at the output end of the data driver does not exceed the predetermined high and low levels even if the output end is shorted to the opposite logic level.
U.S. Pat. No. 4,216,517 discloses another protection circuit which comprises a current detecting section for detecting a level of a load current of an amplifier, a voltage detecting section for detecting a level of a load voltage across a load, an AND-gate circuit responsive to outputs of the current and voltage sections for detecting a short-circuit of the load, and a control section responsive to an output of the AND-gate circuit for disconnecting the load from the amplifier. This circuit is complex and expensive in construction. Since the digital word generator generates many parallel bits such as 8, 16 or 32 bits, the protection circuits for the data drivers are bulky because the protection circuit is needed for each bit.